1. Field of the Invention
The present invention relates to a zoom lens suitable for use in a television camera, a video camera, a photographic camera, and a digital camera. More particularly, the present invention relates to a small and lightweight zoom lens having a high magnification and high performance.
2. Description of the Related Art
In recent years, opportunities of video shooting have been increasing, because video shooting is conducted with not only conventional video cameras or television cameras for broadcasting but also cameras for still image shooting such as single lens reflex cameras and compact digital cameras. The way of video shooting is not limited to shooting a subject in a fixed composition, but it is sometimes necessary for the photographer to follow or track a moving object to keep it in the image frame during shooting or to perform zooming operation during shooting, as is the case when closing up a plurality of objects during shooting. In such occasions, it is required that focus adjustment be appropriately achieved during zooming. If an object of shooting goes out of focus with a zooming operation, the image is blurred undesirably during zooming. Therefore, it is required for lenses used for video shooting to have an auto-focusing function to keep an object in focus even during zooming operations or a focusing function with which an object can be quickly brought into focus. While opportunities of video shooting have been increasing, it is demanded for zoom lenses to have high magnifications, to be small and lightweight, and to be capable of driving focus with high speed and high accuracy. What is called a rear focus system in which focusing is performed by moving a lens unit closer to the image pickup device than the first lens unit has been developed as a technology for enabling size reduction of zoom lenses and increased focus driving speed. In rear focus systems, the weight of the focus lens unit can be made lighter than that in front focus systems, in which focusing is performed by moving the first lens unit having a large diameter and heavy weight. A further reduction in the zoom lens size can be expected by employing, among others, a rear focus system in which a zoom lens unit also functions as a focus lens unit, because this leads to a reduction in the number of moved units. However, the systems in which a zoom lens unit also serves as a focus lens unit suffer from a problem of change in the angle of field of shooting during focusing and a problem that a desired magnification cannot be achieved by zooming at some focus distances. Japanese Patent Application Laid-Open No. H09-015495 and Japanese Patent Application Laid-Open No. H02-154216 disclose zoom lenses developed paying attention to changes in the zoom ratio with changes in the focus distance, in the case where focusing is performed by moving a zoom lens unit.
Japanese Patent Application Laid-Open No. H09-015495 discloses a five-unit zoom lens employing a rear focus system, in which the second to fourth lens units are zoom lens units, the fourth lens unit is also functions to correct variations in the imaging position with focusing, and the amount of shift of the second lens unit is varied in accordance with the focus distance.
Japanese Patent Application Laid-Open No. H02-154216 discloses a four-unit zoom lens employing a rear focus system, in which focusing is performed by moving zoom lens units, and zoom lens units are moved with focusing so that the zoom ratio is not lowered depending on the object distance.
In the zoom lens disclosed in Japanese Patent Application Laid-Open No. H09-015495, it is necessary in order to achieve a high zoom ratio and to perform focusing by moving a zoom lens unit that the second lens unit is moved over a large distance. Therefore, it is difficult to make the zoom lens small in size and light in weight.
In the zoom lens disclosed in Japanese Patent Application Laid-Open No. H02-154216, it is necessary to provide a plurality of lens units that need to move for focusing, requiring a driving mechanism and a space allowing the driving of the lens units. Therefore, it is difficult to achieve both high zoom ratio and reduction in size and weight.
In order to achieve a high zoom ratio, small size, and small decrease in the zoom ratio at short shooting distances in a compatible manner in a five-unit zoom lens in which the second to fourth lens units are zoom lens units, and the fourth lens unit also functions as a focus lens unit, as is the case with the zoom lens according to the present invention, it is important that the zoom share ratios of the second lens unit and the third lens unit in relation to the zoom ratio of the entirety of the zoom lens be set appropriately.
Moreover, in order to achieve reduction in the size of the focus lens unit and reduction in the amount of movement of the focus lens unit for focusing, it is important that the relationship among the focal lengths of the second to fourth lens units and the relationship among the lateral magnifications of the fourth and subsequent lens units be set appropriately.